Organic light-emitting devices based on molybdenum oxide (MoOx) doped 4,4′-bis(carbazol-9-yl)biphenyl (CBP) as a p-type doping hole injection layer was demonstrated. The devices comprise the following structure: ITO/MoOx/CBP∶MoOx/CBP/CBP: tris(2-phenylpyridine)iridium(III)(Ir(ppy)3)/4,7-diphenyl-1,10-phenanthroline(Bphen)/LiF/Al, where CBP acts as hole injection layer (HIL) and hole transport layer (HTL) as well as the host metarial of emitting layer (EML). The simple structure also has the the benefit of lowing the injection and transpotring barrier of holes. With the thickness of HIL increasing, the current density of device increases, indicating the p-type doping layer has a good performance of enhancing the holes injection. The performance of the device can be improved through optimizing the thickness of HIL and HTL. The maximum current efficiency of optimized device is 29.8 cd/A, which can be attributed to the more balanced distribution of carrier in EML. It is noteworthy that the efficiency roll-off of optimized device is only 17.7% between maximum and 20 000 cd/m2, comparing to that of conventional device (62.1%). The undoping CBP layer in optimized device can help to extend the distribution zone of excitons, reducing the possibility of triplet-triplet annihilation and triplet-polaron annihilation, this can be accounted for the improved efficiencies and reduced efficiency roll-off.